WO 2010/061099 A2 (Parrot SA) describes in particular one such drone and how it can be piloted by means of a multimedia player or telephone having a touch screen and an incorporated accelerometer, e.g. a cell phone of the iPhone type, or a multimedia player or tablet of the iPod Touch or iPad type (trademarks registered by Apple Inc., USA). Those appliances include the various control members needed for detecting piloting commands and for bidirectional exchange of data with the drone via a wireless link of the WiFi (IEEE 802.11) or Bluetooth (registered trademarks) local network type. In particular, the appliance is provided with a touch screen that displays the image picked up by the front camera, and superposed thereon various symbols that enable commands to be activated merely by the user making contact with a digit on the touch screen. This display also makes “immersed” piloting possible in which the user does not pilot the drone by looking at the drone itself, but rather makes use of the camera image as though the pilot were on board the drone.
The drone may be used for racing games that consist in following a path that is defined like a slalom by single or double gates or posts, and by a finish line. WO 2008/056049 A1 (Parrot) describes such a quadricopter racing game that consists in going round the circuit defined by pylons and by two virtual rings through which the player must cause the quadricopter to pass.
In order to win the race, it is essential to go fast. And in order to save time, it is necessary to turn around the posts as closely as possible while conserving a maximum amount of kinetic energy, i.e. while traveling relatively fast.
The same applies if the game involves about-turn maneuvers, figures-of-eight, etc.
With present quadricopters, these maneuvers require the user to be skilled and experienced because the mode of piloting requires the user to use several different controls in combination in order to perform these figures.
More precisely, the drone is piloted directly by the user by means of the following in combination:                firstly signals delivered by the inclination detector of the appliance: for example in order to cause the drone to move forwards the user inclines the appliance about the corresponding pitching axis, and in order to cause the drone to shift to right or to left the user inclines the same appliance relative to its roll axis; and        secondly controls that are available on the touch screen, in particular an “up/down” control (corresponding to a throttle control) and a “turn right/left” control (causing the drone to pivot about a yaw axis).        
Thus, if the motors are controlled in such a manner as to cause the drone to tilt or “dive” downwards (to be inclined with a pitching angle), then the drone will move forwards at a speed that increases with increasing angle of inclination; conversely, if the drone takes a “nose-up” position (in the opposite direction), its speed will progressively slow down and reverse, accelerating in the opposite direction. Likewise, an inclination about the roll axis (the drone tilting to right or to left) will give rise to linear movement of the drone in horizontal translation to the right or the left.
This type of horizontal speed control in the forward/backward and left/right directions obtained solely by acting on the speeds of rotation of the four motors is characteristic of a quadricopter.
This differs in particular from speed control systems for other types of aircraft (airplane, single- or dual-rotor helicopter) such as those described for example in JP 2006/312344 A or US 2004/245378 A1, which implement servo-mechanisms for mechanically controlling the positions of ailerons or rudder control surfaces, or for controlling the cyclic pitch of a helicopter rotor in order to modify the angle of incidence of blades so as to enable a horizontal speed component to be created.
The drone described in above-mentioned WO 2010/061099 A2 is also provided with a command for taking up a stationary point (automatic stabilization): when the user lets go of all of the controls on the remote control appliance, the drone stops moving and stays in a stationary point in a manner that is entirely automatic.
Using the available controls, the user can control the path followed by the drone by combining i) movements in XY translation (forward/backward and left/right) by controlling the inclination of the drone about its pitching and roll axes, and ii) movements in pivoting by controlled turning of the drone about its yaw axis. In this discussion, altitude control (up/down) is ignored, and it is assumed that the drone follows its path at constant altitude.
These two types of movement (in XY translation and in pivoting) may be performed simultaneously, but doing so requires a certain amount of skill. A novice will tend to separate the controls by: applying XY piloting to the drone using a left thumb and tilting the appliance; and then letting the drone take up a stationary position by letting go with the thumb; then using the turning control by moving a right finger in order to change direction (thereby turning the XY frame of reference associated with the drone); and then continuing XY piloting in this new direction, etc.
This first method of operation (with controls that are dissociated) does not enable the drone to be piloted quickly.
Another technique consists in piloting the drone in XY mode only along the set path. However under circumstances the front camera in the drone no longer points in the forward direction of the drone, and the visual feedback on the screen of the appliance cannot be used for piloting purposes.
In any event, even when piloting with simultaneous and combined controls, a fast tight turn gives rise to a non-negligible amount of centrifugal force that even an experienced user finds practically impossible to perceive and compensate once the speed exceeds a certain level, thus giving rise to a significant rise of side-slip or of oversteer of the drone with a consequence departure from the path of the race track.
In order to make a turn with combined controls, the user needs to control three degrees of freedom of the appliance simultaneously: pressing a left thumb on the screen while tilting the appliance about its two axes (for controlling the drone in roll and in pitching), while also moving a right finger sideways (for yaw control of the drone)—and while ignoring any up/down control.